Method and apparatus for real time clock (RTC) brownout detection

ABSTRACT

A method and apparatus for real time clock brownout detection. A low power real time clock (RTC) operates continuously to keep time in a global positioning system (GPS) receiver while some receiver components are powered down. In various embodiments, a brownout detector circuit detects a loss of RTC clock cycles. If a loss of RTC clock cycles exceeds a predetermined threshold such that the RTC is not reliable for GPS navigation, an RTC status signal so indicates.

RELATED APPLICATIONS

This application is a continuation-in-part of copending U.S. patentapplication Ser. No. 10/021,119, entitled “Calibrated Real Time Clockfor Acquisition of GPS Signals During Low Power Operation”, filed Oct.30, 2001, which is hereby incorporated by reference.

FIELD OF THE INVENTION

This invention is generally related to Global Positioning System (GPS)receivers. More particularly, it is related to accurately detecting whena real time clock has become inaccurate due to power brownout.

BACKGROUND

The Global Positioning system (GPS) is a collection of twenty-fourearth-orbiting satellites. Each of the GPS satellites travels in aprecise orbit about 11,000 miles above the earth's surface. A GPSreceiver locks onto at least three of the satellites to determine itsprecise location. Each satellite transmits a signal modulated with aunique pseudo-noise (PN) code. Each PN code is a sequence of 1023 chipsthat are repeated every ms consistent with a chip rate of 1.023megahertz (MHz). Each satellite transmits at the same frequency. Forcivil applications, the frequency is known as L1 and is 1575.42 MHz. TheGPS receiver receives a signal that is a mixture of the transmissions ofthe satellites that are visible to the receiver. The receiver detectsthe transmission of a particular satellite by correlating the receivedsignal with shifted versions of the PN code for the satellite. If thelevel of correlation is sufficiently high so that there is a peak in thelevel of correlation achieved for a particular shift and PN code, thereceiver detects the transmission of the satellite corresponding to theparticular PN code. The receiver then uses the shifted PN code toachieve synchronization with subsequent transmissions for the satellite.

GPS employs a unique time keeping system. GPS time is kept in terms ofseconds and weeks since Jan. 6, 1980. There are 604,800 seconds perweek. Therefore, GPS time is stated in terms of a time of week (TOW) anda week number. TOW ranges from 0 to 604800, corresponding to the numberof seconds in a week. The week number started with week zero on Jan. 6,1980 and is currently in excess of one thousand weeks. The TOW can havea fractional part, particularly when oscillators provide a resolution of1/32,768^(th) of a second (an oscillation frequency of 32 kilohertz, orkHz), or when the GPS time is computed from range measurements relativeto a specific clock epoch. GPS time can have accuracy on the order of afew tens of nanoseconds. GPS time is fundamental to the GPS system.

During the initial determination of position of the GPS receiver unit, a“cold start” process is initiated. For a cold start, the GPS receiverbegins the acquisition process without knowledge of GPS time, GPSposition or ephemeris data for the GPS satellite orbits. Therefore, theGPS receiver unit searches for all satellites over a wide range ofpossible frequencies. In some situations, almanac data is also unknownfor the GPS satellites. Eventually, after many seconds, at least foursatellite signals are acquired. The satellites' PN encoded signalsidentify each of the satellites and each satellite transmits ephemerisdata. Ephemeris data includes precise orbital information, for exampleorbital location as a function of GPS time, for that satellite.

If some information is known prior to acquisition, the time to acquiresufficient information from the GPS satellites for navigation cantypically be reduced. For example, a “warm start” process may be used ifalmanac data, approximate GPS time and approximate receiver positionallow approximate satellite locations and Doppler shifts to becalculated. A “hot start” process may be used if the ephemeris,approximate GPS time and approximate receiver position are known so thatapproximate satellite locations and Doppler shifts can be calculated andthe time to collect ephemeris data can be avoided. However, a completesix-second sub-frame of data from at least one satellite is required inorder to establish time with sufficient accuracy to compute a navigationsolution.

The GPS receiver unit determines its distance from each satellite bydetermining the code phase of the transmission from each satellite. Thecode phase (CP) is the delay, in terms of chips or fractions of chips,that a satellite transmission experiences as it travels theapproximately 11,000-mile distance from the satellite to the receiver.At each satellite, the time of transmission of each PN chip iscontrolled down to a few nanoseconds. Consequently, knowledge of preciseGPS time allows the GPS receiver unit to know exactly what chip of asatellite's waveform is being transmitted at any given time. If thearrival of a given chip at a receiver is measured relative to a localtiming epoch, such as the T20 epoch, then the propagation time of thatchip from the satellite to the GPS receiver unit can be measured asaccurately as GPS time at that T20 epoch is known. If the propagationtimes from each of four satellites are measured relative to the same T20epoch, then the GPS receiver unit can solve for the location of thereceiver in three-dimensional space, along with the error in the valueof GPS time at the reference T20 epoch.

The GPS receiver unit precisely determines the distance to the satelliteby multiplying the time delay by the velocity of the transmission fromthe satellite. The GPS receiver unit also knows the precise orbits ofeach of the satellites: Updates of the locations of the satellites aretransmitted to the receiver by each of the satellites. This isaccomplished by modulating a low frequency (50 Hz) data signal onto thePN code transmission from the satellite. The data signal encodes thetime-dependent positional information for the satellite and the timeerrors in its on-board clock in the ephemeris data sub-frames. Precisetime of each satellite's transmission is given in each six-second datasub-frame relative to a reference chip at the start of the nextsub-frame.

Conceptually, the receiver uses the estimated range from a satellite todefine a sphere around the satellite upon which the receiver must belocated. The radius of the sphere is equal to the range to the satellitethe receiver has determined from the code phase. The receiver performsthis process for at least three satellites. The receiver derives itsprecise location from the points of intersection between the at leastthree spheres it has defined. Measurements from three satellites aresufficient if the receiver knows the altitude at its location. When thealtitude is unknown, measurements from four satellites are required sothat altitude can also be solved for, along with latitude, longitude andthe error in the local clock measurement epoch (e.g., GPS time at theT20 epoch).

The detection of the signals from each satellite can be accomplished inaccordance with a GPS signal detector that is disclosed in, for example,but not limited to, U.S. patent application Ser. No. 09/281,566,entitled “Signal Detector Employing Coherent Integration”, filed on Mar.30, 1999, which is incorporated herein by reference. A signal detectoras disclosed therein may use a correlation mechanism, for example amatched filter, and a coherent integration scheme to detect theappropriate satellite signals.

Once the satellite signals are detected, the low frequency 50 Hz datathat is modulated onto the PN code signal received from the satellite isdecoded to determine the precise location of the GPS receiver unit.Conventional location determination processes require several seconds tocomplete. These conventional schemes typically run continually, thusconsuming valuable processor resources. This is especiallydisadvantageous in the case of GPS receiver unit with very limited powerresources, such as a portable GPS receiver unit. Portable GPS receiverunits may be designed such that selected components may be shut off, orpowered down, during periods when the user is not querying the GPSreceiver unit for location information. When the user (or an automatedprocess) queries the GPS receiver unit, the GPS receiver unitreactivates the powered down components and reacquires satellite data todetermine the current location. If the user's location has not changedsignificantly, and/or if the shut down period has been sufficientlyshort, it may be possible to reacquire the previous satellite signalsand achieve nearly immediate correlation of the code phase data (ratherthan the several seconds to minutes associated with the hot, warm orcold start procedures). Nearly immediate correlation of the code phasedata saves several seconds, thereby saving a substantial amount of thelimited power available in a portable GPS receiver unit.

However, reacquisition of the satellite signals with nearly immediatecorrelation of the code phase data requires precise time keeping duringthe period the receiver is off. More particularly, the GPS oscillatorand timing system must maintain accuracy of the various clocking signalsin the GPS receiver unit to within ±0.5 ms to avoid losing track ofwhich PN code period within the overall GPS signal structure thereceiver expects to receive at reacquisition. This 0.5 ms criterioncorresponds to one half of a 1 ms code period. In addition, movement ofthe GPS receiver unit introduces error that may be associated withtiming of the PN code signals. If the accuracy of the clocking signalsplus the error introduced by movement of the GPS receiver unit can bemaintained to within ±0.5 ms of the incoming PN code signals, the timeconsuming and power consuming process of determining location using thehot, warm or cold start procedures may be avoided because the GPSreceiver unit matching filters can immediately lock onto the fourpreviously acquired satellite PN code signals and know which PN codeperiod of the signal structure has been acquired. Otherwise, the hot,warm or cold start procedures must be used, depending on the priorinformation (e.g., almanac, ephemeris, GPS time, and receiver position)that was preserved while selected receiver components, or the entirereceiver, were powered down.

Typically, a conventional real time clock (RTC) circuit may be used tomaintain rough GPS time while the rest of the GPS circuitry is off.Typical RTC circuits may maintain accuracy of a few seconds overextended periods. Such accuracy is adequate for hot and warm starts.However, the accuracy of a conventional real time clock degrades rapidlybelow the ±0.5 ms level due to poor stability and temperaturecharacteristics of typical low cost, low power RTC circuits. Therefore,even after a very brief time, a hot start is required.

Maintaining accuracy of the various clocking signals in the GPS receiverunit to within ±0.5 ms (one half of a 1 ms code period) is not possiblewith a conventional GPS oscillator and timing system if the oscillatoris powered down between navigation updates. However, since the GPSoscillator and the associated timing system consume significant power,powering down these components is very desirable in a portable GPSreceiver unit to conserve power resources.

Under some circumstances, the real time clock may stop altogether due topartial or total loss of the local power source. If the RTC is notoperating at all, it is evident on start-up that a cold start procedureshould be used to acquire satellites. Under other circumstances, the RTCmay seem normally operational on start-up, but may be inaccurate becauseit has experienced partial power loss, or a brownout condition thatcaused the RTC to miss cycles. For example, a battery used to power theRTC may provide inadequate power levels because it is near the end ofits life or because it is subjected to temperatures beyond its operatingrange. This is especially problematic when the time from the RTC istransferred to the GPS clocking scheme to support rapid acquisition. Ifincorrect RTC time is relied on, incorrect range measurements result.Using incorrect range measurements in a navigation solution results inan incorrect position calculation.

One prior RTC failure detector includes a circuit correctly sets astatus flip-flop when the RTC backup power is applied. Usually, thisbackup power is a small battery. Hence, the circuit can accuratelydetect when the backup battery is replaced. This is a relatively uselessfeature. The user knows the battery is being replaced. A set up routinemay be invoked after battery replacement to set the time.

Prior art methods of RTC failure detection that essentially monitorvoltage levels are particularly inadequate when the battery is near itsend of life or when the battery is subjected to wide variations intemperature. For example, the GPS receiver may be placed in a car in acold environment. The battery voltage and current capability may declinein this condition so that the RTC oscillator stops. The user may thentake the receiver, place it in an inside jacket pocket and take a hike.The receiver warms up enough that the battery recovers its capacity andthe oscillator restarts. When the user attempts to use the receiver, thereceiver makes the usual checks. The RTC appears to be running, becausetime is incrementing. The, battery backed RAM (usually on the samebattery as the RTC) has good checksums because the RAM retains itscontents to much lower voltages than the RTC oscillator needs foroperation. The RTC oscillator failure FF indicates good status, becausethe voltage did not fall below the reset threshold and because the logicmay retain its valid state at lower voltages than the oscillatorrequires for operation. Hence, the receiver tries to use the RTC value,assuming it is good, and produces an incorrect solution because the timewas in fact in error. The receiver takes longer to produce a solution,or worse yet, continues to produce bad solutions.

In theory, if the status flip-flop failure detection voltage, thresholdcould be set accurately, the failure would be detected. This isdifficult for a number of reasons. One wants to set the threshold as lowas possible so that the battery life is maximized. This means thethreshold must be precise and that it must respond to differentoscillator requirements for oscillation. These different conditions canbe a function of the particular crystal, the temperature and circuitparameter variations over manufacturing process variations and so on.Hence, some margin in the threshold has to be provided, shortening theuseful battery life. Even with a margin, some failure events may occuron a statistical basis.

It is desirable to have a method and apparatus for GPS navigation can beoperated to conserve power resources by powering down selectedcomponents when they are not in use, yet can quickly acquire satelliteson start-up using a real time clock that operates continuously. It isfurther desirable to have an RTC clock failure detection circuit thatreliably detects oscillator failure without compromising battery life.

SUMMARY OF THE DISCLOSURE

A low power real time clock (RTC) is operated continuously in a GlobalPositioning System (GPS) receiver unit. Power is conserved in the. GPSreceiver unit by shutting down selected components during periods whenthe GPS receiver unit is not actively acquiring satellite informationused to calculate the location of the GPS receiver unit. A K32(typically a nominal 32,768 Hz) oscillator residing in a low power timekeeping circuit accurately preserves GPS time when the selectedcomponents are shut off. The K32 oscillator generates the RTC or lowpower clock. The terms low power clock and RTC are used interchangeablyherein.

A method and apparatus for determining whether the RTC is accurateenough to be used on start-up is disclosed. In one embodiment, actualloss of RTC clock cycles, such as during a brownout episode, isdetected. In one embodiment, an output of an RTC clock oscillator ishalf-wave rectified and placed on the input to a resistor-capacitor (RC)circuit with a calculated RC time constant. The output of the RC circuitis placed on one input of a voltage comparator. A reference voltage isplaced on the other input of the voltage comparator. If the RTCoscillator misses a predetermined number of cycles, the output voltageof the RC circuit on the voltage comparator decays and the comparatordetects the loss of clock cycles, which is reflected on the voltagecomparator output.

BRIEF DESCRIPTION OF THE DRAWINGS

The components in the figures are not necessarily to scale, emphasisinstead being placed upon illustrating the principles of the invention.

FIG. 1 is a block diagram of an exemplary environment for operation of aglobal positioning system (GPS) receiver;

FIG. 2 is a block diagram of an embodiment of a GPS receiver;

FIG. 3 is another block diagram of the embodiment of the GPS receiver;

FIG. 4 is another block diagram of the embodiment of the GPS receiver;

FIG. 5 is a block diagram of an embodiment of a brownout detectioncircuit;

FIG. 6 is a flow diagram illustrating a brownout detection process ofone embodiment; and

FIGS. 7A, 7B, and 7C are a flow chart illustrating one embodiment of aprocess that includes using an RTC clock signal to update a GPS clocksignal, and determining whether or not an estimated GPS time issufficiently accurate to acquire position of a GPS receiver.

DETAILED DESCRIPTION

FIG. 1 illustrates an example environment for operation of a globalpositioning system (GPS) receiver. FIG. 1 shows a GPS receiver unit 100and four GPS satellites 102, 104, 106 and 108. Each satellite 102, 104,106 and 108 is transmitting to the GPS receiver unit 100. Satellite 102is moving towards the GPS receiver unit 100 along the line of sight(LOS) 110 at a velocity ν_(a) ⁺; satellite 104 is moving away from theGPS receiver unit 100 along the LOS 112 at a velocity ν_(b) ⁻ andsatellite 106 is moving away from the GPS receiver 100 along the LOS 106at a velocity ν_(c) ⁻. Consequently, assuming a carrier wavelength of λ,the transmission from satellite 102 experiences a positive Doppler shiftof $\frac{v_{a}^{+}}{\lambda};$the transmission from satellite 104 experiences a negative Doppler shiftof $\frac{v_{b}^{-}}{\lambda};$and the transmission from satellite 106 experiences a negative Dopplershift of $\frac{v_{c}^{-}}{\lambda}.$

Satellite 108 is similarly moving away from the GPS receiver unit 100along the LOS 116 at a velocity ν_(d) ⁻. Information provided by thefourth satellite 116 may be used in some applications to determine theerror in the altitude value of the receiver if it is not knownbeforehand. The four satellites must have adequate geometry in order toprovide measurements capable of solving for latitude, longitude,altitude and time error. Range measurements from more than the minimumquantity of four visible satellites may be required to solve for thefour unknown quantities when satellite geometry is poor.

FIG. 2 is a block diagram of a GPS receiver unit 100 according to oneembodiment. The GPS receiver 100 includes radio frequency (RF)functionality shown here residing on an RF chip 103. The GPS receiverunit 100 further includes baseband functionality shown here residing onbaseband chip 105. Various components that perform various functionswill be described in certain arrangements herein, but the invention asdisclosed contemplates alternative arrangements. For example, thebaseband chip 105 may include a navigation processor 210 and a memorydevice 220, as shown. In other embodiments, the navigation processor andthe memory device may not reside on the baseband chip 220, but maycommunicate with the baseband chip 220 through, for example, aperipheral interface. In yet other embodiments, all of the componentsshown and functionalities described reside on one chip.

The RF chip 103 includes a GPS oscillator 204, which is a high accuracyoscillator used to keep GPS time. The following is an overview ofgeneral operation of the GPS receiver unit 100 according to oneembodiment. Components named in the following overview will be shown anddescribed below. Power is conserved in GPS receiver unit 100 by shuttingdown selected components, including the GPS oscillator 204, duringperiods when the GPS receiver unit is not actively acquiring satelliteinformation used to calculate the location of the GPS receiver unit. AK32 (typically a nominal 32,768 Hz) oscillator residing in a low powertime keeping circuit accurately preserves GPS time when the selectedcomponents are shut off.

The GPS oscillator 204 generates a clock signal, referred to as the M11clock signal, that is used to accurately determine GPS time based uponsignals detected from the plurality of satellites. An edge aligned ratiocounter continuously monitors the K32 and M11 clock signals with freerunning counters, and when an edge of the K32 clock signal aligns withan edge of the M11 clock signal within a predetermined small tolerance,the K32 and M11 counter values are latched. Since the GPS timinggenerator that produces the T20 epochs is driven by the M11 clock, thefree running M11 counter can also be latched at a T20 epoch to establisha relationship between that counter and the T20 epoch. Thus, the GPSreceiver unit 100 is able to correlate the timing and the rates of theK32 clock signal and the GPS M11 clock signal with the T20 timing epoch.The correlated timing and rates of the K32 clock signal, the GPS M11clock signal and the T20 epoch are provided to the navigation processor210 so that a sufficiently accurate estimate of GPS time at a T20 epochis calculated to allow determination of the PN code periods in thesignal structures of acquired satellite PN code signals.

During operation of the GPS receiver unit, frequencies of the local GPSoscillator and the K32 oscillator are detected at various operatingtemperatures such that a temperature/frequency is defined for bothoscillators. The data for both temperature/frequency tables are storedin a memory.

Selected components residing on the GPS receiver unit, including the GPSoscillator, are then shut down (deactivated) to conserve power. The lowpower time keeping circuit remains on. Periodically, after apredetermined period of time, the system is powered up in response to awake-up command generated by an alarm unit. The K32 clock signal fromthe low power time keeping circuit is recalibrated based upon the actualoperating temperature of the K32 oscillator and data from the K32 clocktemperature/frequency table. Thus, the K32 clock rate is periodicallyupdated to more accurately track GPS time.

At a particular point in time, a navigation update is performed inaccordance with the requirements of the particular system application.The periodically recalibrated K32 clock signal and data from the GPSclock temperature/frequency table are used to set the M11 clock signalrate and GPS time. Positions of the GPS satellites are then estimatedsuch that the real GPS time can be quickly determined from the receivedsatellite signals. Once the precise GPS time is determined from thedetected satellite signals, the M11 and K32 signals are latched togetherand correlated with the real GPS time at a T20 epoch, as describedabove, to further improve and update their temperature calibrationtables. The selected components are then shut off once again to conservepower.

The process described above is repeated as necessary so that accurateGPS time is maintained by the low power time keeping circuit. When auser of the GPS receiver unit requests position information, the GPSreceiver unit determines position from the GPS satellites more quickly,because the GPS satellite positions and ranges are estimated with a highdegree of precision based on more accurate time keeping. That is, thepower consuming and time consuming process of detecting sub-frame dataand determining sub-frame timing to set the GPS time accurately enoughto estimate the ranges to the GPS satellites using conventionalprocesses is avoided.

Referring again to FIG. 2, the RF chip 103 and the baseband chip 105communicate through a system interface 109. In one embodiment, thesystem interface 109 is a serial peripheral (SPI) interface, but inother embodiments, the system interface could be any adequate messagingscheme. The RF chip 103 receives signals from satellites in view via anantenna 107. The satellite signals are sampled and sent to thenavigation processor as a serial stream on the SIGN/MAG line. Thebaseband chip 105 and its components operate with an ACQCLK signal thatis generated from a GPS oscillator crystal, and typically has afrequency that is a multiple of F₀. Various other signals are exchangedvia the system interface as show. A power up (PWRUP) signal is sent tothe RF chip 103 to power up the powered down components of the RF chip103 prior to acquisition and navigation. An SPI_CLK signal is sent tothe RF chip 103 from the baseband chip 105 for synchronization. Datalines SPI_DI and SPI_DO carry data back and forth. A chip enable signal(RFRST) is sent to the RF chip 103 on the RFRST line and a reset signal(SRESET_N) is sent to the baseband chip 105 on the RFRST line. In otherembodiments, various different protocols are used to exchangeinformation between the RF chip 103 and the baseband chip 105.

FIG. 3 is a block diagram illustrating selected components of the GPSreceiver unit 100, including a low power time keeping circuit 200. GPSreceiver unit 100 includes at least a radio 202, the local GPSoscillator 204, temperature sensor 206, a plurality of GPS signalprocessor channels 208 (1 through N), the navigation processor 210,matched filter 212, A/D converter 214, local GPS clocks generator 216,edge aligned ratio counter 218, memory 220 and low power time keepingcircuit 200. Memory 220 further includes the wake-up alarm logic 222 andan allocated portion for the GPS clock Low Power Time Keeping (LPTK)Circuit error temperature/frequency error table 224. FIG. 3 is generallylimited to illustrating those components relevant to the operation andfunctioning of the invention. Other components, not shown, are includedin the GPS receiver unit 100. These components are omitted because adiscussion of the operation and functionality is not necessary for thedisclosure.

Radio 202 detects a plurality of GPS signals from a plurality ofsatellites, such as, but not limited to, satellites 102, 104, 106 and108 of FIG. 1. In one embodiment the radio 202 selects the GPS L1 band(1575.42 MHz). However other embodiments may select other suitablesignals. Radio 202 also receives a timing signal from local GPSoscillator 204, via connection 226. The timing signal, in oneembodiment, is generated by a crystal (not shown) residing in the localGPS oscillator 204 that oscillates at substantially 10.949 mega-hertz(MHz), and is referred to as the M11 clock signal. Other embodiments mayemploy a local GPS oscillator operating at a different frequency clocksignal without departing substantially from the operation andfunctionality of the invention.

The received GPS signals and the M11 timing signal are provided to theplurality of GPS signal processors 208 and matched filter 212. Each oneof the plurality of GPS signal processors 208 corresponds to aparticular signal channel. FIG. 3 indicates that there are N GPS signalprocessors. For instance, an exemplary embodiment of the GPS receiverunit 100 may employ twelve GPS signal processors (N=12) that areconfigured to process in parallel twelve signal channels.

The signal processors 208 and matching filter 212 receive a sequence ofpre-positioning commands from the navigation processor 210, viaconnection 230, that indicate specific GPS PN codes that each signalprocessor is to search for. Information provided by navigation processor210 may also include Doppler correction values, GPS oscillator errorcorrection values, PN code phase information and/or other relevantinformation regarding the incoming satellite signals.

In one embodiment, the matched filter 212 determines the current PN codephase of a detected signal and provides the information to the signalprocessors 208 to allow the signal processor channel to more rapidlyacquire that signal. When one of the signal processors 208 detects asignal on a channel such that the PN code, code phase and frequencycorrection matches that of one of the incoming GPS signals, the GPSsignal processor synchronizes to and tracks the incoming satellitesignal. Another embodiment employs only the matched filter 212 todetermine position (although with a lesser degree of accuracy since thematched filter 212 determines the current code phase of a signal at apoint in time and does not continually track it). Current embodiments ofthe matched filter also permit fast multiplexing of the matched filterthat does allow continuous, accurate tracking of all acquired satellitesignals.

The matched filter 212 and/or the GPS signal processors 208 provide codephase information regarding the acquired signals to the navigationprocessor 210, via connections 234 and/or 232, respectively. Navigationprocessor 210 then calculates the position of the GPS receiver unit 100after sufficient information from at least four GPS satellite signalshas been provided by the matched filter 212 and/or the GPS signalprocessors 208. The location information is then output to an interfacesystem (not shown) so that a user may learn of the position of the GPSreceiver unit 100.

The local GPS oscillator 204 provides a signal having a predefinedoscillation frequency. For example, but not limited to, the oscillationfrequency of a crystal (not shown) residing in one embodiment of thelocal GPS oscillator 204 is configured to equal 10.949296.875 megahertz(MHz). Here, the precise nominal value of the oscillation frequencyequals 137 F₀/128. F₀ is a fundamental parameter of the GPS system equalto 10.23 MHz. The GPS L1 frequency of the received GPS signals is 154F₀. The chip rate of the Clear/Acquisition GPS PN codes used incommercial system s is F₀/10. One embodiment of the GPS oscillator 204is referred to as outputting an M11 clocking signal, where the term“M11” corresponds to the 137 F₀/128 frequency of 10.949296.875 MHz.Other signals of the GPS system, including frequencies and codes used bymilitary receivers, are also related to F₀.

The local GPS oscillator 204 provides the M11 clocking signal, viaconnection 234, to the local GPS clocks generator 216. Local GPS clocksgenerator 216 derives a plurality of clock signals from the M11 clockingsignal. These clocks correspond to the local GPS time-base. Ofparticular interest, one of the plurality of clocks is known as thelocal timing epoch, the T20 clock. The T20 clock derives its name fromthe fact that it is 20 ms between clock ticks. Many of the code phasesmeasured in the GPS signal processors 208 and the matched filter 212 arereferenced to a common T20 epoch. Selected clocking signals generated bythe local GPS clocks generator 216 are provided to the GPS signalprocessors 208 and the matched filter 212 over connection 236.

The low power time keeping circuit 200, described in detail below,provides a clocking signal to the edge aligned ratio counter 218, viaconnection 252. The clocking signal rate, in one embodiment, is providedby a crystal oscillating at substantially 32.768 kilohertz (kHz), and isreferred to as the K32 clock signal. Also, the low power time keepingcircuit 200 provides information to the navigation processor 210(connections not shown). Typically, the information provided to thenavigation processor 210 by the low power time keeping circuit 200 is anestimate of the GPS time at a T20 epoch. Other embodiments may employ adifferent frequency clock signal without departing substantially fromthe operation and functionality of the invention.

A brownout detection circuit 235, shown and described in more detailbelow with reference to FIGS. 5 and 6, detects loss of RTC clock cycles.The brownout detection circuit 235 detects a situation in which loss ofRTC clock cycles have made the RTC too inaccurate to use on start-up andnotifies the navigation processor accordingly, as described in moredetail below.

The edge aligned ratio counter 218 provides input to the local GPSclocks generator 216 (via connection 244), to the matched filter 212(via connection 246), and to the low power time keeping circuit 200 (viaconnection 248). For convenience of illustration, connections 244, 246and 248 are illustrated as separate connections. However, one or more ofthese connections could be implemented as a single connection. The edgealigned ratio counter 218 also provides information to the navigationprocessor 210 via connection 250. The edge aligned ratio counter 218continuously counts and monitors the K32 and M11 clock signals, and whenan edge of the K32 clock signal aligns with an edge of the M11 signal,within a predetermined small tolerance, the K32 and M11 counter valuesare latched. At the time of latching, the edge aligned ratio counter 218provides a signal to the local GPS clocks generator 216 so that thecurrent T20 clocking count is latched to relate the K32 and M11 countsto the T20 epoch. In the same manner, the edge aligned ratio counter 218provides a signal to the low power timekeeping circuit 200, viaconnection 248, that causes the current low power timekeeping circuit200 estimate of GPS time to be latched. Thus, the GPS receiver unit 100is able to correlate the timing and the rates of the K32 clock signaland the GPS M11 clock signal with the T20 epoch and the current lowpower timekeeping circuit 200 GPS time. When the correlated timing andrates of the K32 clock signal, the GPS M11 clock signal, the low powertime keeping 200 GPS time and the T20 epoch count are provided to theNavigation processor 210, the low power time keeping circuit 200estimate of GPS time at a T20 epoch can be calculated and the relativerates of these two clocks can be estimated from counter ratios of thetwo clocks in the edge aligned ratio counter 218. In order to estimatethe relative clock frequency, two sets of counter values from successiveedge alignment events are differenced and the ratios of the differencestaken.

Note that the rate and timing phase of all clocks can be accuratelyrelated. The EARC free running M11 counter and T20 epoch generator areboth driven by the M11 clock. Hence, latching the M11 counter at a T20epoch relates the counter values and T20 epoch times. The RTC time andthe EARC free running K32 counter are both driven by the K32 clock.Hence, latching the K32 counter at a RTC alarm event relates the countervalues and the RTC times. The RTC has an alarm circuit that produces apulse, useful as a latching signal, at a desired RTC time. Using theEARC to latch the free running K32 and M11 counters at an edge alignmentevent relates the K32 and M11 counter values. Differencing therespective K32 and M11 counter values from two alignment events allowsthe ratio of the K32 and M11 clock rates to be related. Finally, whenGPS signals are being tracked, calculation of the GPS solution providesthe precise GPS time at a T20 epoch and the rate of the T20 epochsrelative to GPS time. Hence, the K32 and M11 clock rates can be relatedto GPS clock rate and the RTC and T20 epoch times can be related to GPStime.

One skilled in the art will appreciate that the above describedoperation of the GPS receiver unit 100 is intended as a generaldescription of one system used by an embodiment of a GPS receiver unit.Not all GPS receiver unit components are described or illustrated, assuch components may not necessarily relate to the invention. Thus, thedescription of the above-described components residing in the GPSreceiver unit 100 is generally limited to describing the operation andfunctionality of those components to the extent necessary for theunderstanding of the invention. Furthermore, a GPS receiver unit orother processor systems employing the invention may have the componentsshown in FIG. 3 connected in a different order and manner than shown inFIG. 3, or may not include all of the component shown in FIG. 3, or mayinclude additional components connected in some manner with thecomponents shown in FIG. 3. Any such variations in GPS receiver unit ora processor system that utilizes the invention are intended to be withinthe scope of this disclosure and be protected by the accompanyingclaims.

Temperature sensor 206 detects the operating temperature of the localGPS oscillator 204, via connection 238. The sensed temperatureinformation is then provided to the A/D converter 214 over connection240. A/D converter 214 converts the sensed operating temperatureinformation into a suitable format and provides the information to thenavigation processor 210, via connection 242. Temperature sensor 206 andA/D converter 214 may be implemented using well-known components andtechniques employed in the art of detecting temperatures. Thetemperature sensing functions performed by temperature sensor 206 and/orA/D converter 214 may be implemented with any type of electronic, solidstate and/or firmware type temperature sensors or means commonlyemployed in the art of sensing temperatures. Such a temperature sensoremployed in the invention is implemented by a combination of softwareand firmware using components and techniques commonly employed in theart of sensing temperatures. Detailed operation of temperature sensor206 and AID 214, including their individual components, are notdescribed in detail other than to the extent necessary to understand theoperation and functioning of invention. One skilled in the art willrealize that the temperature sensor 206 and the A/D converter 214 may beimplemented using a variety of well known devices without departingsubstantially from the functionality and operation of the invention.

Navigation processor 210 processes the received temperature informationsuch that a frequency error in the GPS oscillator signal due to theoperating temperature of the local GPS oscillator 204 is determined. Anexemplary process for determining this frequency error employs a tablehaving temperature and frequency error information for a range ofoperating temperatures. In one embodiment, the GPS clocktemperature/frequency error table 224 resides in a non-volatile memory220. Initially, a frequency/temperature error algorithm, such as apolynomial representation of the frequency error as a function oftemperature of a typical oscillator crystal, is employed to approximatethe temperature related frequency error. As the GPS receiver unit 100 isoperated over time, the portion of the temperature/frequency error table224 for the GPS clock data is filled with more accurate information ofthe frequency error at particular operating temperatures for the localGPS oscillator 204 based upon measurements of frequency error based onGPS satellite range and range rate measurements at various operatingtemperatures. Solution of the GPS navigation equations allows adetermination of receiver spatial velocity and local oscillatorfrequency error (rate of change of GPS time error), as well as spatialposition and GPS time error. The oscillator frequency error sodetermined is paired with the current oscillator temperature as a newupdate point in the temperature/frequency error table 224.

Prior to entering the navigation mode, the receiver uses thetemperature/frequency error table 224 to aid in the satelliteacquisition process. Upon receiving the current operating temperature,the navigation processor 210 looks up the table information for the GPSclock residing in the temperature/frequency error table 224. The actualoperating temperature of the local GPS oscillator 204 is correlated withthe data in the temperature/frequency error table 224 to estimate thefrequency error in the signal generated by the local GPS oscillator 204.This GPS clock frequency error information is provided to the GPS signalprocessors 208 and the matched filter 212, via connection 230.Alternatively, when the temperature/frequency error table 224 is onlypartially filled out and does not contain sufficient data for the exactcurrent operating temperature, a frequency/temperature errorextrapolation or interpolation algorithm may be used to estimate theerror in the GPS oscillator signal due to the operating temperature ofthe local GPS oscillator 204. This algorithm makes use of the points inthe table at the nearest temperatures to the current operatingtemperature along with the shape of the nominal temperature vs.frequency curve of the type of GPS clock oscillator crystal in use.

FIG. 4 is a block diagram illustrating additional detail of the GPSreceiver unit 100. Low power time keeping circuit 200 further includesat least a K32 oscillator 302, a signal latch 304, a temperature sensor308 and a low power clock, or real time clock (RTC) 306.

K32 oscillator 302 outputs the RTC clock signal, also referred to as theK32 clock signal, having a frequency substantially equal to 32.768 kHz,via connection 310. Since the K32 oscillator 302 provides a K32 clocksignal having a time resolution of 32768 Hz, which equals approximately30 microseconds, the K32 oscillator 302 provides a clocking signalhaving a frequency well within the ±0.5 ms resolution of a single PNcode period.

The RTC clock signal is sent to the brownout detection circuit 235 onconnection 310. The brownout detection circuit 235 is explained in moredetail below.

K32 oscillator 302 provides its output K32 clock signal to the counterin the low power clock 306 and to the edge aligned ratio counter 216.When the edge aligned ratio counter 216 determines that an edge of theK32 clock signal aligns with an edge of the M11 signal, within apredetermined small tolerance, a latch signal is provided to the signallatch 304, via connection 248. The current value of the low power clockcounter 306 is latched in signal latch 304 when the edge alignmentsignal is received, via connection 248. The latched value in signallatch 304 is provided to the navigation processor 254, via connection316. The signal on connection 316 is thus the low power clock signal, orRTC clock signal. The edge aligned ratio counter 216 provides thelatched values of the M11 and K32 counters in the edge aligned ratiocounter at the alignment event epoch to the navigation processor 210.Because the T20 epochs can be directly related to the GPS oscillator M11clock (not shown), the M11 counter value in the edge aligned ratiocounter 216 can be related to the K32 counter value in the low powerclock 306 as an offset of a specific number of integral M11 clock ticks.The number of clock ticks is integral (has no fractional clock tickcomponent) because the counter values were all acquired when the K32 andM11 clock edges were aligned within a small (negligible) window oferror. Because the low power clock 306 has been closely calibrated tothe time and rate of the GPS system time, knowing the value of the lowpower clock 306 and an offset to a specific T20 epoch in the local GPStime line allows the GPS time of the low power time keeping circuit 200to be transferred accurately to the T20 epoch. Since all GPS measurementsignal processing is related to T20 epochs, the measurements can now bemade relative to an accurate local GPS time estimate.

The K32 oscillator 302 and the low power clock 306 are, relatively, verylow-power consuming devices, particularly when compared to the selectedcomponents residing in the GPS receiver unit 100 that are powered downin a manner described below. Furthermore, the K32 oscillator 302 and thelow power clock 306 are commercially available and relativelyinexpensive. Alternatively, and preferably, the K32 oscillator 302 andlow power clock 306 can be integrated into the GPS device 100 to provideeven lower cost, smaller size and more accurate time-transferperformance.

As illustrated in FIG. 4, a temperature sensor 308 detects the operatingtemperature of the K32 oscillator 302, via connection 318. The sensedtemperature information is then provided to the A/D converter 214, viaconnection 320. A/D converter 214 converts the sensed temperatureinformation into a suitable format and provides the K32 operatingtemperature information to the navigation processor 210, via connection242. Temperature sensor 308 may be implemented using well-knowncomponents and techniques employed in the art of detecting temperatures.The temperature sensing functions performed by temperature sensor 308may be implemented with any type of electronic, solid state or firmwaretype temperature sensor or means commonly employed in the art of sensingtemperatures. Such a temperature sensor 308 employed in the invention isimplemented by a combination of software and firmware using componentsand techniques commonly employed in the art of sensing temperatures.Detailed operation of temperature sensor 308 is not described in detailother than to the extent necessary to understand the operation andfunctioning of the invention. One skilled in the art will realize thatthe temperature sensor 308 may be implemented using a variety of wellknown devices without departing substantially from the functionality andoperation of the invention. Any such embodiments of temperature sensor308 that are employed as a part of the invention are intended to bewithin the scope of this disclosure and to be protected by theaccompanying claim.

A portion of the temperature/frequency error table 224, included inmemory 220, is used to store temperature/frequency data for the K32oscillator 302. Navigation processor 210 calculates a frequency errorassociated with the signal from the K32 oscillator 302 based upon thecurrent operating temperature of the K32 oscillator 302, much like thelocal GPS oscillator 204 frequency error described above. As the GPSreceiver unit 100 is operated over time, the temperature/frequency errortable 224 is filled with more accurate information of the frequencyerror at particular operating temperatures for the K32 oscillator 302based upon measurements of frequency error at various operatingtemperatures. Unlike the case of the M11 GPS oscillator, the navigationprocessor 210 does not have a direct means of measuring the error in theK32 oscillator. However, while navigating, the navigation processor 210can accurately estimate the error in the M11 signal from the GPSoscillator 302 and then use the edge aligned ratio counter 216 totransfer the GPS time from a T20 epoch to a low power clock value at aK32 tick having a known offset of a near-integral number of M11 ticksfrom a T20 epoch. Since GPS range measurements are made relative to T20epochs, the T20 epochs have GPS time errors that are accurately knownwhen navigation solutions are available from GPS measurements.Transferring the accuracy of the T20 epoch GPS time to the low powerclock 306 during navigation calibrates the K32 clock signal at thecurrent K32 oscillator 302 temperature.

Alternatively, when data for the K32 oscillator 302 in thetemperature/frequency error table 224 is only partially filled out, anembodiment of the invention employs a frequency/temperature erroralgorithm, such as a polynomial representation of the frequency error asa function of temperature of a typical K32 oscillator crystal 302, toapproximate the temperature related frequency error of the K32 clocksignal based on extrapolation or interpolation from the nearesttemperature value or values having a valid table value. Such analgorithm mathematically correlates frequency errors and operatingtemperatures.

To conserve power, many of the GPS receiver unit 100 components, andother components of the GPS device, are shut off. During periods of timewhen the components are shut off to conserve power, referred to as thesleeping period or the sleep mode, the invention accurately keeps trackof GPS time, as described below. Thus, when the GPS receiver unit 100leaves the sleep mode, such as in response to a “wake-up event” or inresponse to another signal indicating that position is to be determined,the GPS time is accurately maintained such that a minimal amount of timeis required to track the GPS satellites to determine the location of theGPS receiver unit 100.

For example, but not limited to, the local GPS oscillator 204, radio202, local GPS clocks generator 216 and/or GPS signal processors 208 mayhave been powered down by the navigation processor 210 to conservepower. Powering down the selected components, when the components arenot required to actively process incoming GPS satellite signals, reducesoverall power consumption by the GPS receiver unit 100, therebyextending limited power source life in a portable GPS receiver unit 100.Typically, components that consume relatively large amounts of powerduring operation are selected for the power down. It is understood thatthe designer of the GPS receiver unit 100 selects the components thatare to be shut off during the power down process. Since there are agreat number of components residing in the GPS receiver unit 100 thatmay be powered down, many of which have not been described, one skilledin the art will appreciate that a detailed description and inventory ofthe components that may be powered down are too numerous to convenientlylist and describe in detail. Any such combinations of components thatare powered down in accordance with the invention are intended to bewithin the scope of this disclosure and to be protected by theaccompanying claims.

Powering down the selected components in conventional GPS receiversresults in the loss of GPS satellite signal tracking. When suchconventional GPS receivers power up after loss of the GPS satellitesignals, several seconds are required for the reacquisition of the GPSsatellite signals and/or the establishment of GPS time of sufficientaccuracy for navigation using those signals. The time required forsatellite signal and time reacquisition in the conventional GPSreceivers results in a corresponding power use. Therefore, the low powertime keeping circuit 200 that accurately maintains GPS time during thesleeping period enables a GPS receiver unit 100 to more quicklyreacquire the GPS satellite signals, thereby saving power resources.

The wake-up command is provided to the GPS receiver unit 100 on aperiodic basis. The time between the periodic wake-up commands isdetermined based upon the specific architecture or application of theGPS receiver unit 100 in which the low powered time keeping circuit 200has been implemented. The time between wake-up commands is selected suchthat the time error accumulated between the local replica PN code phaseestimated by the navigation processor 210 after the power down periodand incoming PN codes is less than or equal to ±0.5 ms of the actual PNcode phase of the incoming GPS satellite signals. In the event that thePN code estimated by the navigation processor 210 exceeds the ±0.5 mscriteria, the navigation processor 210 initiates a conventional processto acquire GPS satellite information. Typically, the receiver 100 mustestimate the likely error accumulation and choose the correct algorithmaccordingly. Since the algorithm chosen may be too optimistic (using thefast acquisition rather than the conventional acquisition), thenavigation processor 210 must verify the time accuracy hypothesis bycomparing the resulting position and time error solution with the apriori assumed values. If the combined time and time-equivalent positionerrors in fact exceeded ±0.5 ms, the resulting solution will typicallydiffer from the a priori values by recognizably large errors. If theerror is not greater than ±0.5 ms, GPS time has been maintained withsufficient accuracy by the low power time keeping circuit 200.

The alarm unit 324 performs the functionality of implementing theperiodic wake-up commands, also referred to as periodic navigationupdates. The alarm unit 324 includes at least an alarm register 326 anda comparator 238. In one embodiment, prior to shutdown, the navigationprocessor 210 executes the wake-up alarm logic 222 to define theperiodic times that the alarm unit 324 is to wake up the GPS receiverunit 100. In another embodiment, the time periods are predefined.

These time periods defining when the wake-up commands are issued areprovided to the alarm register 326 via connection 330. In oneembodiment, the time periods are defined in GPS time units (TOW and weeknumber). In another embodiment, another suitable time period such asreal time is used to define time periods.

Once the GPS receiver unit 100 is placed into a sleep mode, the alarmunit 324 monitors the K32 clock signals provided from the low powerclock 306 (that is not shut down during sleep mode) to determine thecurrent sleep mode time. The comparator 328 compares the current sleepmode time with the periodic times that the alarm unit 324 is to wake upthe GPS receiver unit 100. When the current sleep mode time and theperiodic times match, the alarm unit 324 generates a periodic wake-upcommand. This periodic wake-up command initiates a power up of thecomponents that were powered down during the sleeping period.

In one embodiment, the periodic wake-up command initiates a power upusing special purpose, dedicated hardware. For example, the wake-upcommand actuates one or more power switches such that the componentsthat were powered down during the sleeping period are provided power. Inanother embodiment, the wake up command is provided to the navigationprocessor 210 such that the wake-up alarm logic 22 is executed to wakeup the components that were powered down during the sleeping period.

The alarm unit 324, and its associated components, may be implementedusing well-known components and techniques employed in the art ofgenerating wake-up commands. Detailed operation of the alarm unit 324,and its associated components, are not described in detail other than tothe extent necessary to understand the operation and functioning ofinvention. One skilled in the art will realize that the alarm unit 324,and its associated components, may be implemented using a variety ofwell known devices without departing substantially from thefunctionality and operation of the invention. Any such embodiments ofthe alarm unit 324, and its associated components, that are employed asa part of the invention are intended to be within the scope of thisdisclosure and to be protected by the accompanying claims.

As alternative embodiment may employ another suitable processor (notshown) that performs the power down and power up functions. Such aprocessor and its related components would not be powered down duringthe sleeping period. Such an alternative processor would be configuredto generate the periodic wake-up command. The processor may be acomponent of another system (not shown in FIGS. 3 and 4) residing in theGPS receiver unit 100, or be a stand alone dedicated processor residingin the GPS receiver unit 100. Any such alternative embodimentimplemented in a GPS receiver unit 100 to perform the functionality ofgenerating periodic wake-up commands is intended to be within the scopeof this disclosure and to be protected by the accompanying claims.

Also, the user may instruct the GPS receiver unit 100 to power up thecomponents upon receiving a manually initiated wake-up command thatcorresponds to a positional query. For example, when the user of the GPSreceiver unit 100 wants to be informed of the present location of theGPS receiver unit 100, the use initiates a manual wake-up command. Asuitable means are provided for the user to query the GPS receiver unit100. The means to manually initiate a wake-up command may be implementedusing well-known components and techniques employed in the art ofactivating devices. Detailed operation of a means to manually initiate awake-up command is not described in detail other than to the extentnecessary to understand the operation and functioning of invention. Oneskilled in the art will realize that the means to manually initiate awake-up command may be implemented using a variety of well known deviceswithout departing substantially from the functionality and operation ofthe invention. Any such embodiments of the means to manually initiate awake-up command that are employed as a part of the invention areintended to be within the scope of this disclosure and to be protectedby the accompanying claims.

When the wake-up command initiates start up, the clocking signals (e.g.,T20 epochs) provided by the local GPS clocks generator 216 will not bewithin the accuracy required to enable the GPS receiver unit 100 toperform a position update without first reacquiring satellite signalsand collecting a six-second sub-frame of date to re-establish a commonlocal GPS time frame for GPS satellite range measurements. However, ifthe PN code estimated by the navigation processor 210 after the end ofthe power down period, based upon time kept by the low power timekeeping circuit 200, and incoming PN codes can be maintained to be lessthan or equal to ±0.5 ms of the actual PN code time of the incoming GPSsatellite signals, GPS satellite signals are quickly re-acquired andmeasurements relative to a common local GPS time frame can be taken andused in navigation without performing the conventional process ofacquiring GPS satellite signals and establishing a common time frame.

Prior to the power down, the time and rate relationships between theK32, M11 and GPS clock signals were known. By maintaining the K32 clocksignal accuracy, the K32 clock signal is used by the edge aligned ratiocounter 218 to latch the K32 clock signal and M11 signal, therebyrecalibrating the M11 signal and the T20 epochs derived from it. Thus,the GPS oscillator 204 is recalibrated. The navigation processor 210then sets up the matched filter or signal processor channels to acquirethe PN code phases of satellites calculated to be visible. The matchedfilter or signal processor channel set up takes advantage of the GPSoscillator versus temperature data previously stored to compensate forfrequency error in the GPS oscillator. When code phase measurements areobtained, these values are converted from a knowledge of which chip in aPN code period is currently being received to which chip in the overallGPS signal structure is being received. This conversion is made by usingthe assumed current GPS time and receiver position to calculate which PNchip of the overall signal structure should be arriving at the receiverand assuming the chip actually arriving is the instance of this chip ina PN code period that is closest to the one that should be arriving. Ifthe hypothesis that the combined local GPS time estimate and thetime-equivalent receiver position error is correct, the translation intothe overall GPS signal structure will be correct and a consistent set ofGPS range measurements will be determined. In other words, if the errorof the PN code estimated by the navigation processor 210 after the endof the power down period (after leaving the sleep mode) and incoming PNcodes is less than or equal to ±0.5 ms of the actual PN code time of theincoming GPS satellite signals, position information is correctlyupdated. The computed position and time must be compared to the a prioriestimates to verify that the error was in fact less than ±0.5 ms. If theverification fails, a six-second sub-frame must be collected toestablish the common time frame for measurement.

The position and time error information acquired by the GPS receiverunit 100 is then used to update the M11 and K32 clock errors. Both theGPS oscillator 204 and the K32 oscillator 302 are updated for frequencyerror. The K32 low power clock 306 is updated for correct GPS time. TheGPS receiver unit 100 is then placed back into a sleep mode to conservepower. The above-described process is then repeated when the nextwake-up command is received. This periodic updating, therefore,conserves power while maintaining the accuracy of the clock signals suchthat the GPS unit does not have to reacquire satellite positions usingconventional processes.

Whenever the wake-up command is received, the K32 clock signal is usedto update the M11 clocking signal. However, the K32 clock signal derivedfrom the K32 oscillator 302 is subject to some error in that the K32oscillator 302 frequency is temperature dependent. That is the K32oscillator 302 frequency is different for different operatingtemperatures. In one embodiment, the temperature sensor 308 senses theoperating temperature of the K32 oscillator 302. The navigationprocessor 210 compares the detected operating temperature of the K32oscillator 302 with information residing in the LP clocktemperature/frequency error table 322. Based upon the time between theperiodic wake-up commands and the sensed operating temperature of theK32 oscillator 302, an error correction factor is determined such thatthe K32 time and rate are corrected to account for the operatingtemperature of the K32 oscillator 302. That is, the time of the K32clock signal is corrected by the error factor to account for the actualoperating temperature of the K32 oscillator 302. As described above, inone embodiment, the data in the LP clock temperature/frequency errortable 322 is based upon historical data collected during actualoperation, and is therefore highly accurate.

Once the K32 clock signal is recalibrated, time associated with the M11signal is recalibrated. In one embodiment, the temperature sensor 206senses the temperature of the GPS oscillator 206. The navigationprocessor 210 compares the detected operating temperature of the GPSoscillator 206 with information residing in the GPS clocktemperature/frequency error table 224. Software then uses this ratecorrection as time progresses to scale the interval between T20 epochsbased on the M11 clock to maintain the correct GPS time estimates ateach epoch. Further, the initial value of GPS time at the T20 epochsjust after wake-up is determined by transferring the GPS time from theK32 low power clock 306 to the M11 based T20 epochs using the edgealigned ratio counter 216 as previously described. Since the M11oscillator was off during the sleep period, its elapsed time cannot bescaled as the K32 low power clock 304 elapsed time was. As describedabove, in one embodiment, the data in the temperature/frequency errortable 224 is based upon historical data collected during actualoperation, and is therefore highly accurate. Then, when the K32 clocksignal (now temperature corrected) is used to update the M11 clocksignal (also temperature corrected), the PN code estimated by thenavigation processor 210 after the power down period is less than orequal to ±0.5 ms of the actual PN code time of the incoming GPSsatellite signals.

In an alternative embodiment, a wake-up event may be programmed to occurmore frequently than that required for navigation updates. Such wake-upevents would only serve the purpose of sampling the current temperatureof the K32 oscillator. Based on the average of the temperatures of thecurrent and prior wake-up events, the elapsed time between the twowake-up events is scaled to correct for the change in temperature. Theresulting correction can be either applied to the low power clock 306 orelse simply stored in a non-volatile memory until future calculationsrequire use of the correction. Furthermore, this alternative may beupgraded to provide for a dynamic wake-up period. That is, the timebetween wake-up commands may be changed depending upon the particularoperating conditions encountered. If the total temperature change in theK32 oscillator 302 during the power down period exceeds the predefinedthreshold, the time period between wake-up commands is decreased by asuitable amount of time. On the other hand, if the total temperaturechange is less than the predefined temperature threshold, the intervalof time between wake-up commands is increased by some suitable amount oftime. Thus, the power consumed to maintain an accurate temperature isminimized relative to the requirements of the current environment oftemperature dynamics.

As an enhancement of the foregoing alternative, navigation processor 210may consider the total change in operating temperature of the K32oscillator since the last periodic wake-up command and the currentperiodic wake-up command. If the temperature change exceeds a predefinedthreshold, the navigation processor 210 may immediately initiate anavigation update process to reacquire GPS satellite signals to ensurethat the integrity of the low power clock 306 is maintained withinacceptable limits.

FIG. 5 is a block diagram of one embodiment of the brownout detectioncircuit 235 according to one embodiment. The brownout detection circuit235 includes a detection circuit 237 and a status circuit 239. The RTCclock signal is input to the detection circuit 237 on the line 310. TheRTC clock signal is half-wave rectified by the diode shown. Thehalf-wave rectified RTC clock signal is input to a resistor-capacitor(RC) circuit that includes components R₁, R₂, and the capacitor shown.An output of the RC circuit on line 241, referred to herein as the decayvoltage, is one input to a voltage comparator 281.

As long as the RTC oscillator is operating, the detector 237 maintainssome average DC voltage at the comparator input. The other input to thevoltage comparator 281 is a reference voltage 243, which is the outputof a voltage divider formed by V_(DD) and resistor R₃. The voltagereference is sized for the lower range of battery voltage near end oflife. This also ensures that the filtered, rectified clock voltage willclimb above this threshold unless the clock is off for a substantialnumber of cycles. To accomplish this, the RC time constant is maderelatively long. This makes the detection circuit 237 insensitive toexact battery voltage. The long time constant also reduces the powerconsumption of the circuit 237, because relatively little energy isrequired from the oscillator to the detector circuit 237.

The status circuit 239 includes the flip-flop 283. The flip-flop 283indicates a low or high logic value on its output 259. As justexplained, the output of the detection circuit clears the flip-flop 283when the RTC is not good. The flip-flop 283 is set to indicate the RTCis GOOD by a signal on the set input 257.

When power is first applied, the detector circuit 237 and flip-flop 238will respond in less time than it takes for the oscillator to power up,and thus the detected voltage at the input of the comparator 281 willexceed the threshold. Hence, the status circuit will be reset to NOTGOOD, when the battery has been removed and is replaced.

If the RTC oscillator clock stops long enough, the comparator inputvoltage will fall below threshold and clear the flip-flop to indicatethe RTC clock is NOT GOOD.

The status flip-flop 238 is set indicate the RTC clock is GOOD by thenavigation processor or other processor that has the responsibility toinitially acquire GPS satellites and produce a time and positionsolution without being able to use the RTC time. Once the processor hasproduced the time and position solution, the processor sets the RTC,verifies that the RTC is correctly propagating time, and finally setsthe flip-flop 238 to indicate the RTC clock is GOOD. As long as the RTCoscillator continues to operate and produce the RTC clock, the voltagewill remain above threshold and the RTC status will remain GOOD.

If the RTC oscillator fails for some period, the voltage at thecomparator input will gradually decay. After a sufficient number ofmissed clocks, the flip-flop 238 is set indicate the RTC clock is NOTGOOD. The flip-flop 238 remains in this state until the processor againreestablishes time. One of the chief objectives of the detection circuit235 is to protect against oscillator stoppage due to battery end of lifeand/or temperature variations. If the issue is end of life, the backupbattery is likely to remain below the threshold required foroscillation. If the issue is temperature, the time constants associatedwith temperature are relatively slow. Furthermore, once the oscillatorhas stopped due to the battery experiencing low temperature, theoscillator will likely require a higher voltage to restart than thevoltage (and current) being supplied when it stopped. Hence, a timeconstant that requires even thousands of cycles is acceptable.

The detection circuit 235 can be written to and read from in variousmanners in different embodiments. For example, in some embodiments, thedetection circuit 235 resides on the RF chip 103, and in otherembodiments resides on the baseband chip 105. The output 259 of thestatus circuit 239 may be read using a command according to a bus orinterface protocol, or may be directly monitored. Similarly, the setinput 257 of the status circuit 239 may be toggled by any software orhardware mechanism according to the specific architecture of anembodiment.

For example, a microprocessor bus interface may read and write theflip-flop 283. In this situation, reading the flip-flop 283 may requirethe peripheral bus strobe to be active, the write line to be inactive,the peripheral select decode to activate an appropriate select signal,and a local RTC block decode to assert a “RTC_GOOD” signal. Reading andwriting the flip-flop 283 according to a bus protocol is contemplated,for example, when the brownout detection circuit resides on the basebandchip.

In another embodiment, the RTC oscillator and the brownout detectorcircuit 235 reside on the RF chip. This allows a quieter environment forthe oscillator, enhancing the ability to accurately calibrate the RTCoscillator and to locate this oscillator closer to the temperaturesensor for calibration purposes. In this case, the interface to theflip-flop 283 would be different. For example, a message decode from aserial IO port would select the flip-flop 283 for reading and latch thisbit into a message, subsequently clocked out over the port to aprocessor on the baseband chip.

Many circuit variations are within the scope of the disclosed brownoutdetection circuit. The particular circuit components shown are but oneembodiment to perform the desired function. Many other circuits arepossible and practical for particular environments. For example, thecapacitor in the detector circuit must be very small for mixed signalintegration. Thus, the simple RC time constant may be replaced by someadditional electronics to amplify the effective capacitance. Similarly,the asynchronous set interface to the flip-flop 283 may actually be asynchronous set interface from a processor bus. These are known circuittechniques that enhance the concept by reducing circuit size or powerconsumption.

FIG. 6 is a flow chart showing the operation of the brownout detectioncircuit 235 according to an embodiment. At start-up of the GPS receiver100, as shown at 602, the navigation processor 210 reads the RTC at 602.This RTC time is transferred to the EARC at 604. At 608, the status ofthe RTC is checked by reading the output 259 of the detector 235. If theRTC is GOOD, the navigation processor proceeds to use the transferredRTC time to begin acquisition at 610. If the RTC is NOT GOOD, one courseof action is for the navigation processor 210 to proceed with a coldstart at 612. At 614, the navigation processor 210 produces a time andposition solution. With the time solution, the navigation processor 210set the RTC at 616. The navigation processor 210 verifies that the RTCclock is running at 618. If the RTC clock is verified, the navigationprocessor 210 sets the RTC status GOOD at 620 by sending a signal to thestatus circuit 239. If the RTC clock is not verified, the navigationprocessor 210 again attempts to verify that the RTC clock is running at618.

The brownout detection process illustrated in FIG. 6 is one embodimentof a process that is performed along with other processes describedherein. For example, FIGS. 7A, 7B and 7C illustrate an embodiment of aprocess that is contemplated to be performed with the process of FIG. 6.Referring now to FIGS. 7A, 7B and 7C, flow chart 400 illustrates anembodiment of a process that includes using the K32 clock signal toupdate the M11 clock signal. The process of flow chart 400 furtherincludes determining whether or not the estimated GPS time issufficiently accurate to acquire position of the GPS receiver unit 100.If the time error between the PN code estimated by the navigationprocessor 210 during the power down period and incoming PN codes is lessthan or equal to ±0.5 ms of the actual PN code time of the incoming GPSsatellite signals, the K32 clock signal and the M11 clock signal areupdated. The process of flow chart 400 further includes updating the M11clock signal associated with the GPS oscillator 204 and the K32 clocksignal associated with the K32 oscillator 302 with detected GPSsatellite information. After the update, the GPS receiver unit 100 thenreturns to a sleep, or powered down, mode.

The flow chart 400 further illustrates an embodiment of the wake-upalarm logic 222. In some alternative implementations, the functionsdescribed may occur out of the order noted in the flow chart 400, thefunctions described may occur concurrently, some of the functionsdescribed may be eliminated, or additional functions may be included.

The process begins at block 402 when a wake-up command is generated bythe alarm unit 324. Alternatively, the process may also begin when auser queries the GPS receiver unit 100 to provide position information(a “navigation update”).

At block 404, a determination is made whether the reason for thepower-up was a wake-up command or a position query from the user. If thereason for the power-up was the generation of the wake-up command by thealarm unit 324 such that the GPS receiver unit 100 is to update the K32based time maintained by the low power timekeeping circuit 200, then theprocess proceeds to block 406. However, if the reason for the power-upis to provide location information in response to a position query fromthe user, the GPS receiver unit 100 initiates a navigation update byproceeding to block 422.

At block 406, selected components that are employed in the recalibrationof the K32 clocking signals as described below, are powered up. Othercomponents of the GPS receiver unit 100 are not powered up at block 406to conserve power. For example, the GPS receiver unit 100 may include adisplay (not shown) that indicates to the user at least determinedposition information. If the GPS receiver unit 100 is performing aperiodic navigation update, the user may not be interested in eitherknowing that the device is performing a navigation update or in knowingthe position information. Thus, the display (not shown) is not poweredup at block 406, thereby conserving power.

At block 408, temperature sensor 308 measures the temperature of the K32oscillator 302. At block 410, an average temperature is determined forthe K32 oscillator 302 during the time that the GPS receiver unit 100was in the sleep mode. At block 412, the K32 based time maintained bythe low power time keeping circuit 200 is accessed. Based upon a timeerror at block 414, based upon information in the temperature/frequencyerror table 224, a correction factor as described above is applied tothe K32 based time determined. This correction factor is then used tocorrect the K32 based time maintained by the low power time keepingcircuit 200 at block 416.

In one embodiment, the time for the next wake-up command is determinedat block 418. Accordingly, the wake-up time is updated in the alarmregister 326. Alternatively, other embodiments employ a predefined timeinterval between the periodic wake-up commands and/or provide periodicwake-up commands from other components.

At block 420, the selected powered-up components (at block 406) arepowered down. Since the K32 based time maintained by the low power timekeeping circuit 200 has been updated, these selected components arepowered down to conserve power resources. The process proceeds back toblock 402 to await the next wake-up command or a position query from theuser.

If a position query is received block 404, the GPS receiver unit 100understands that it is to accurately determine the location of the GPSreceiver unit 100 and to indicate the location to the user, and theprocess proceeds to block 422. That is, the user desires a navigationupdate.

Accordingly, components of the GPS receiver unit 100 described below arepowered-up at block 422. Components associated with the updating of theM11 based time are powered-up at block 422. For example, the radio 202,the GPS oscillator 204, the temperature sensor 206, the navigationprocessor 210, the match filter 212, the A/D converter 214, the localGPS clocks generator 216, the edge aligned ratio counter 218 and/or thememory 220 are repowered.

Furthermore, the GPS receiver unit 100 may include additionalcomponents, not associated with the updating of the M11 based time, thatare powered-up at block 422. For example, a display (not shown) andassociated circuitry may be used to indicate to the user the determinedposition information. Thus, the display must be powered-up. In contrast,the display did not need to be powered-up at block 406 because thelocation information was not displayed during the update of the K32based time as described above (blocks 406-416). In one embodiment, theseadditional components are powered up concurrently with the abovedescribed components at block 406.

In another embodiment, the powering up of these additional components isdelayed until the navigational update is completed. Accordingly, block422 would be shown as two separate blocks, with the powering up of theadditional components shown with a new block inserted at a later pointin the flow chart 400. After the GPS receiver unit 100 has determined anupdated position, these additional selected components are powered upsuch that the updated position is indicated to the user. For example,the GPS receiver unit 100 may include a display (not shown) andassociated circuitry that indicates to the user at least determinedposition information. Such an alternative embodiment delaying repoweringof these additional selected components only when a position update isrequested is particularly advantageous for conserving power. That is, ifthe selected additional components are not required for therecalibration of the clocks and the associated navigation update,maintaining the selected components in a sleep mode when a wake-upcommand is received further conserves power.

At block 424, temperature sensor 308 measures the temperature of the K32oscillator 302 and corrects the K32 based time maintained by the lowpower time keeping circuit 200 by correcting the time using a correctionfactor determined from the temperature/frequency error table 224 usingthe process described above in blocks 408-416. That is, the K32 basedtime is corrected for any temperature/frequency deviations occurringduring the sleeping period.

At block 426, the updated K32 based time is transferred to the M11 basedtime by the edge line ratio counter 216. Thus, the GPS receiver unit 100has powered up its components and used the corrected K32 based time fromthe low power time keeping circuit 200 to accurately update GPS timefrom the M11 clocking signal provided by the GPS oscillator 204.However, in one embodiment, error in the M11 clocking signal may haveoccurred due to temperature changes of the GPS oscillator 204.Accordingly, at block 428, temperature sensor 206 measures thetemperature of the GPS oscillator 204. At block 430, the current GPSoscillator 204 temperature is determined. At block 432, the M11frequency error is determined from the temperature/frequency table.

At block 436, the updated T20 epoch is used to estimate the position andthe Dopplers of the visible GPS satellites 102, 104, 106 and/or 108.Based upon the estimated position of the visible satellites 102, 104,106 and/or 108, the GPS receiver unit 100 employs the matched filter 212or the GPS signal processors 208 to measure the PN code phase (modulo 1ms) for the visible satellites 102, 104, 106 and/or 108 at block 438.Then, at block 440, the estimated T20 epoch is used to calculate theexpected current full PN code phase, as a time of week (TOW), for eachof the satellites 102, 104, 106 and/or 108. That is, the GPS receiverunit 100 has used the updated M11 clocking signal from the GPSoscillator 204 to accurately estimate a modulo 1 ms PN code phase tocalculate an expected complete PN code phase as a time of week.

At block 443 the full code phase is corrected to match the measured PNcode phase (modulo 1 ms). At block 444 the navigation solution iscomputed based upon the estimated corrected full PN code phase. Next, atblock 446, the computed navigation solution is compared with theprevious navigation solution in units of time.

At block 448, a determination is made whether the calculated position ofthe GPS receiver unit 100 has changed by less than ±0.5 ms (less than 1PN code) from the previous navigation solution time. If the determinedchange is greater than ±0.5 ms (the NO condition) the process proceedsto block 450 such that the GPS receiver unit 100 collects an entire 6second sub-frame from each of the GPS satellites 102, 104, 106 and/or108 to establish GPS time. At block 452, the GPS receiver unit 100employs a conventional method to update the navigation solution, therebyaccurately determining the position of the GPS receiver unit 100.

However, if at block 448 the change in position is determined to be lessthan or equal to ±0.5 ms (the YES condition) the GPS receiver unit 100has accurately maintained GPS time with the low power timekeepingcircuit 200. Accordingly, the process proceeds to block 454 such thatthe corrected T20 time is used to update the low power time keepingcircuit 200 M11 time using the edge aligned ratio counter 218 in amanner described above. Thus, the K32 clocking signal is correlated withthe accurately determined GPS T20 time in preparation for the next powerdown period.

In one embodiment, the data residing in the temperature/frequency errortable 224 is updated with the temperature and frequency informationcollected above. That is, this embodiment employs acquired temperatureand frequency data to continuously update the temperature/frequencyerror table 224 data, thereby improving the accuracy of subsequentcorrection factors determined from the temperature/frequency error table224.

At block 458, a determination is made whether or not the GPS receiverunit 100 is to stay on. If the GPS receiver unit 100 is to stay on (theYES condition), the process proceeds to block 460 such that the GPSreceiver unit 100 performs other functions. Such other functions are notdescribed in detail herein as such functions may not necessarily berelated to accurately maintaining time during power down periods. Afterthese other functions have been performed, the process proceeds back toblock 418 such that the next time of the wake-up command is determinedas described above.

If at block 458 a determination is made that there is no reason for theGPS receiver unit 100 to stay on (the NO condition), the processproceeds directly to block 418. That is, the process proceeds to block418 such that the GPS receiver unit 100 is powered down to conserveenergy while the low power time keeping circuit 200 accurately maintainsGPS time.

The above described embodiments of a GPS receiver unit 100 are generallydescribed as updating the K32 clock signal derived from the K32oscillator 302 and the M11 clocking signal derived from the GPSoscillator 204 such that accurate GPS time is maintained during periodswhen the GPS oscillator 204 is powered down. Other embodiments update avariety of other clocking signals associated with the determination oflocation from GPS satellites. Furthermore, the GPS oscillator 204 wasdescribed as providing a signal having an oscillation frequencysubstantially equal to 11 MHz. Similarly, the K32 oscillator 302 wasdescribed as generating a signal having a frequency of oscillationsubstantially equal to 32 kHz. Other embodiments of GPS receiver unitsmay be implemented with a GPS oscillator and/or an oscillator residingin the low power time keeping circuit that have frequencies ofoscillation different from the oscillation frequencies of the GPSoscillator 204 and the K32 oscillator 302. Furthermore, the low powertime keeping circuit was described as providing a clocking signalsubstantially at 32 kHz that was used for maintaining the GPS timeaccuracy during the periods of time that the components were poweredoff. The clocking signals provided from the low power time keepingcircuit 200, in other embodiments, are used to provide clocking signalsto other components residing in a GPS receiver unit. However, suchcomponents are not described in detail other than to the extentnecessary to understand the operation and functionality of theinvention.

In an alternative embodiment, temperature sensors 206 and 308 arereplaced by, or incorporated into, a single temperature sensor suitablylocated so that the operating temperatures of the GPS oscillator 204 andthe K32 oscillator 302 are detected. Such a temperature sensor may befurther configured to provide a signal directly to the navigationprocessor 210. This embodiment reduces the number of components, and mayprovide a corresponding decrease in costs, size and power consumption.

For convenience of illustration in FIGS. 3 and 4, and for convenience ofexplaining the operation and functionality of the invention, processingthe sensed temperatures and calculating the total frequency error in thesignal from the K32 oscillator 302, and the GPS oscillator 204, wasdescribed and shown as being implemented by the execution of logic bythe navigation processor 210, such logic residing as a portion of thewake-up alarm logic 222. Alternatively, the processing could beimplemented by a different processor. Furthermore, the logic forprocessing sensed temperatures and logic for calculating the totalfrequency error in the signal from the K32 oscillator 302 could residein dedicated logic modules (not shown) residing in memory 220, or inanother suitable memory. Additionally, the LP clocktemperature/frequency error table 322 and/or the GPS clocktemperature/frequency error table 224 was shown as residing in memory220 for convenience. Sensed temperatures table 508 could reside in analternative location and/or in a suitable alternative storage medium.Any such alternative implementations are intended to be within the scopeof this disclosure and to be protected by the accompanying claims.

While various embodiments of the invention have been described, variousmodifications available to those of ordinary skill in the art are withinthe scope of this invention, which is defined by the claims.

1. A global positioning system (GPS) receiver system, comprising: a GPSclock that is calibrated to GPS time when the GPS receiver system isnavigating using GPS satellite data, wherein the GPS clock is configuredto be turned off when the GPS receiver system is not navigating; a realtime clock (RTC) that uses significantly less power than the GPS clock,wherein the RTC is configured to keep time when the GPS clock is turnedoff; a brownout detection circuit coupled to the RTC, wherein thebrownout detection circuit is configured to, receive an RTC clocksignal; detect a loss of RTC clock cycles; and output an RTC statussignal that indicates a loss of RTC clock cycles above a predeterminedthreshold.
 2. The GPS receiver system of claim 4, wherein the brownoutdetection circuit comprises: a detection circuit that receives the RTCclock signal and determines whether the RTC clock is losing cycles,wherein the detection circuit is calibrated to determine whether a lossof cycles is above the predetermined threshold; and a status circuitthat stores a signal output by the detection circuit and outputs astatus signal indicating the RTC clock is one of GOOD and NOT GOOD. 3.The GPS receiver system of claim 2, wherein the detection circuitcomprises a resistor-capacitor (RC) time constant component with apredetermined time constant, wherein the RC time constant componentreceives the RTC clock signal and outputs a decayed voltage, wherein alevel of the decayed voltage indicates whether the loss of cycles isabove the predetermined threshold.
 4. The GPS receiver of claim 3,further comprising a navigation processor coupled to receive the statussignal, wherein the navigation processor determines whether to use theRTC clock for acquisition of satellites based on the status signal. 5.The GPS receiver system of claim 4, further comprising an edge alignedratio counter (EARC) coupled to the RTC and to the GPS clock, wherein,on start-up of the GPS receiver system for satellite acquisition, timekept by the RTC clock is transferred to the GS clock using the EARC, andwherein the transferred RTC time is used for acquisition if the statussignal indicates the RTC is GOOD.
 6. A system for global positioningsystem (GPS) navigation comprising: a baseband chip; and a radiofrequency (RF) chip, wherein the RF chip and the baseband chip arecoupled through an interface, and wherein the RF chip comprises: a GPSclock that is calibrated to GPS time when the GPS receiver system isnavigating using GPS satellite data, wherein the GPS clock is configuredto be turned off when the GPS receiver system is not navigating; a realtime clock (RTC) that uses significantly less power than the GPS clock,wherein the RTC is configured to keep time when the GPS clock is turnedoff; and a brownout detection circuit coupled to the RTC, wherein thebrownout detection circuit is configured to detect a loss of RTC clockcycles.
 7. The system of claim 6, wherein the RF chip further comprises:a temperature sensor coupled to the RTC; and an analog to digital (A/D)converter coupled to the temperature sensor.
 8. The system of claim 7,wherein the baseband chip comprises: a navigation processor coupled toreceive signals from the RF chip through the interface, including an RTCstatus signal that indicates whether the RTC clock signal should be usedfor satellite acquisition; an edge aligned ratio counter (EARC) coupledto receive a GPS clock signal and the RTC clock signal and configured toalign respective GPS and RTC clock signals with a high degree ofaccuracy, and to transfer time kept by the RTC clock to the GPS clock;and a memory device coupled to the A/D converter and to the RTC, andconfigured to store a table relating temperature to frequency for theRTC clock.
 9. The system of claim 7, wherein the brownout detectioncircuit comprises: a detection circuit that receives the RTC clocksignal and determines whether the RTC clock is losing cycles, whereinthe detection circuit is calibrated to determine whether a loss ofcycles is above the predetermined threshold; and a status circuit thatstores a signal output by the detection circuit and outputs a statussignal indicating the RTC clock is one of GOOD and NOT GOOD.
 10. Thesystem of claim 9, wherein the detection circuit comprises aresistor-capacitor (RC) time constant component with a predeterminedtime constant, wherein the RC time constant component receives the RTCclock signal and outputs a decayed voltage, wherein a level of thedecayed voltage indicates whether the loss of cycles is above thepredetermined threshold.
 11. The system of claim 7, wherein theinterface comprises a serial peripheral interface.
 12. The system ofclaim 8, wherein the navigation processor sends a command via theinterface to the brownout detection circuit requesting a status of theRTC, and wherein the brownout detection circuit responds by sending anRTC status via the interface.
 13. A system for global positioning system(GPS) navigation comprising: a radio frequency (RF) chip, wherein the RFchip comprises a GPS clock that is calibrated to GPS time when the GPSreceiver system is navigating using GPS satellite data, wherein the GPSclock is configured to be turned off when the GPS receiver system is notnavigating; and a baseband chip, wherein the baseband chip and the RFchip are coupled through a system interface, and wherein the basebandchip comprises, a real time clock (RTC) that uses significantly lesspower than the GPS clock, wherein the RTC is configured to keep timewhen the GPS clock is turned off; and a brownout detection circuitcoupled to the RTC, wherein the brownout detection circuit is configuredto detect a loss of RTC clock cycles.
 14. The system of claim 13,wherein the baseband chip further comprises: a temperature sensorcoupled to the RTC; and an analog to digital (A/D) converter coupled tothe temperature sensor.
 15. The system of claim 14, wherein the basebandchip further comprises an edge aligned ratio counter (EARC) coupled toreceive a GPS clock signal and the RTC clock signal and configured toalign the respective clock signals with a high degree of accuracy, andto transfer time kept by the RTC clock to the GPS clock.
 16. The systemof claim 15, wherein the baseband chip is coupled to a processor and amemory through a peripheral interface, wherein: the memory device iscoupled to the A/D/converter and to the RTC, and is configured to storea table relating temperature to frequency for the RTC clock; and theprocessor is configured to receive signals through the peripheralinterface, including an RTC status signal that indicates whether the RTCclock signal should be used for satellite acquisition.
 17. The system ofclaim 13, wherein the brownout detection circuit comprises: a detectioncircuit that receives the RTC clock signal and determines whether theRTC clock is losing cycles, wherein the detection circuit is calibratedto determine whether a loss of cycles is above the predeterminedthreshold; and a status circuit that stores a signal output by thedetection circuit and outputs a status signal indicating the RTC clockis one of GOOD and NOT GOOD.
 18. The system of claim 17, wherein thedetection circuit comprises a resistor-capacitor (RC) time constantcomponent with a predetermined time constant, wherein the RC timeconstant component receives the RTC clock signal and outputs a decayedvoltage, wherein a level of the decayed voltage indicates whether theloss of cycles is above the predetermined threshold.
 19. The system ofclaim 13, wherein the system interface comprises a serial peripheralinterface.
 20. The system of claim 16, wherein the processor sends acommand via the peripheral interface to the brownout detection circuitrequesting a status of the RTC, and wherein the brownout detectioncircuit responds by sending an RTC status signal via the peripheralinterface.
 21. An apparatus for detecting a loss of clock cycles in aclock signal generating device, the apparatus comprising: a detectioncircuit that receives the a clock signal from the clock signalgenerating device, and determines whether the clock signal generatingdevice is losing cycles, wherein the detection circuit is calibrated todetermine whether a loss of cycles is above the predetermined threshold;and a status circuit that stores a signal output by the detectioncircuit and outputs a status signal indicating the clock signalgenerating device is one of GOOD and NOT GOOD.
 22. The apparatus ofclaim 21, wherein the detection circuit comprises a resistor-capacitor(RC) time constant component with a predetermined time constant, whereinthe RC time constant component receives the clock signal and outputs adecayed voltage, wherein a level of the decayed voltage indicateswhether the loss of cycles is above the predetermined threshold.
 23. Theapparatus of claim 22, wherein: the status circuit comprises a latchdevice; and the detection circuit further comprises a voltage comparatorcoupled to latch device, wherein the voltage comparator compares thedecayed voltage and a reference voltage and outputs a result signal thatresets the latch when the loss of cycles is above the predeterminedthreshold.
 24. A method of determining a status of a real time clock(RTC) in a global positioning system (GPS) receiver, the methodcomprising: receiving an RTC clock signal in a detection circuit;detecting when the RTC is losing clock signals such that the loss ofclock cycles is above a predetermined threshold; storing the status ofthe RTC, wherein the status is one of GOOD and NOT GOOD; if the loss ofclock cycles is above the predetermined threshold, setting the status ofthe RTC to bad; and before using the RTC clock signal for acquiringsatellites, checking the status of the RTC.
 25. The method of claim 24,wherein detecting comprises receiving the RTC clock signal in aresistor-capacitor (RC) circuit with a calculated RC time constant suchthat when the loss of clock cycles is above the predetermined threshold,an output voltage of the RC circuit decays below a predetermined level.26. The method of claim 25, wherein storing the status comprises storinga status bit based on the output voltage level of the RC circuit,wherein a first logic value of the status bit indicates GOOD and asecond logic value of the status bit indicates “bad.
 27. The method ofclaim 26, further comprising, on start-up of the GPS receiver, settingthe status bit to indicate GOOD during an interval when the RTC ispowering up.
 28. The method of claim 27, further comprising: on start-upof the GPS receiver, transferring time kept by the RTC to a GPS clockusing an edge aligned ratio counter (EARC); checking the status of theRTC; and if the status of the RTC is GOOD, using the transferred time toacquire satellites.